A. Field of Invention
The present invention relates to digital communication systems. More specifically, it relates to timing recovery in a baud-rate sampled communication system.
B. Background of the Invention
Communication devices such as modems establish connections at a data rate that is predetermined, or one that is negotiated during an initialization stage. A data rate is typically measured in bits-per-second ("bps"). The number of bits per second is related to the type of signaling, or encoding, that is used to convey the information, and the number of times per second that the transmitted signal changes its value. The number of signal variations per second is called a "baud rate." If the transmitted signal represents one data bit, only two states are needed (e.g., 0 or 1). During the baud time, these states may be represented as one of two voltage levels, two frequencies, varied carrier phases, or other well-known signaling formats. In the case where each baud represents one bit, the terms "baud" and "bps" are equivalent. However, if a signaling element is chosen from a set of elements having more than two elements, it can be used to convey more than one bit of information. For example, in a signal set having four levels, each level may represent 2-bits of information. In this scenario, the signal transmitted during each baud time represents two bits. Thus the bit rate is twice the baud rate. The term "symbol," rather than "baud" is often used to refer to signaling elements.
Several different encoding methods are used to encode signals on a communication device such as a modem. The encoding schemes include Frequency Shift Keying ("FSK"), also called frequency modulation, Amplitude Shift Keying ("ASK"), also called amplitude modulation, Phase Shift Keying ("PSK") also called phase modulation, and Phase and Amplitude Modulation ("PAM"). If a signal's phase shift is measured relative to a previous signal, Differential Phase Shift Keying ("DPSK") may be used where n-bits can be assigned to a signal having one of 2.sup.n phase shifts where the bit rate is n-times the baud rate. For more information see Understanding Data Communications and Networks, by William A. Shay, PWS Publishing Company, Boston, Mass., 1995.
Communication devices such as modems adhere to communication standards that define a baud rate, a bit rate and a modulation scheme. The International Telecommunications Union-Telecommunication Standardization Sector ("ITU-T", formerly known as the CCITT) define standards for communication devices operating over the telephone network, identified by "V.xx" series where "xx" is an identifying number. For example, the ITU-T V.26 defines a communication standard for a baud rate of 1200 baud, a bit rate of 2400 bps and a modulation technique of PSK. A communication device typically contacts another communication device, exchanges signals according to predefined protocols, and then utilizes the appropriate standard for communicating.
When a communication device negotiates a standard for communications, the device is expected to adhere to the standard it negotiated during the communication. For example, if a first communication device negotiates to use V.26 to communicate with a second communication device, then the communication device should use a baud rate of 1200 baud, a bit rate of 2400 bps and a PSK modulation technique to send data.
If a transmitter and receiver do not utilize a common timing reference to demarcate baud intervals, then the receiver typically attempts to recover baud timing information from the received data signal. The received signal may be a baseband voltage waveform directly representing the logical values, or may be a modulated carrier wave, or other form of modulation such as wavelets. When the analog signal arrives at the receiving location, it is often sampled, and processed digitally in order to recover the data carried within the received analog signal. The received signal may be sampled at an interval that results in numerous samples per baud period, but at a minimum, must be sampled at least once per symbol.
As is known in the art, a signal is a time-continuous signal within a time range t if its amplitude is specified for all value of t in that range. If a time-continuous signal is "discretized," that is, if t if represented by a set of discrete values denoted by t.sub.k, where k takes on positive integer values, the signal is defined only at discrete-time instants known as "sampling instants." Signals which are defined only at discrete instants of time are called "discrete-time" or "discrete signals" and are represented by a sequence of numbers. The samples are generally equally-spaced in time. If the spacing is denoted by an arbitrary constant T, the discrete signal can be expressed by x(kT) or x(k), where k represents discrete time and T is the sampling interval, or sampling period.